Algae bloom event, an extreme ecological imbalance that the water environment experiences, changes the phosphorus (P) cycle in the aquatic environment, which makes the lake maintain a long-term eutrophication and frequent algae bloom state. This study compared P form characteristics and bacteria community structures in the aquatic environment of the cyanobacteria area and non-cyanobacteria area of Taihu Lake, aiming to clear the new P cycle pattern disturbed by algae bloom and decomposition processes. Compared with P forms in mediums of the middle of the lake and the east of the lake, there were higher concentration levels of total particulate P (TPP) in water, organic P (OP) in suspended particles, iron bound P (FeP) in sediments and phosphate (PO₄³⁻) in the pore water of Meiliang Bay, the cyanobacteria area. OP form was the dominant P fraction in suspended particles that occupied 69% in particulate total P, but OP proportion in sediments decreased to 26% of sediment total P, which indicated the strong occurrence of OP mineralization in sediments. The higher concentration and proportion of FeP in sediments of Meiliang Bay suggested the intensified effects of algae bloom and decomposition on sediment FeP accumulation. In Meiliang Bay, the positive correlation between Fe²⁺ and PO₄³⁻ in pore water and the higher diffusion fluxes of Fe²⁺, PO₄³⁻ from pore water to overlying water (0.45, 0.65 mg/m²·d) than that in the other lake areas also suggested the intensified effects of algae bloom and decomposition on FeP reductive dissolution in sediments accompanying sediment P remobilization. Moreover, there were higher concentrations of labile sulfide and high relative abundances of iron reducing bacteria (FRBs), sulfate reducing bacteria (SRBs) in sediments of Meiliang Bay. Results suggested that algae bloom event changed the natural P cycle in aquatic environment through intensifying the pathways of sediment OP mineralization, FeP accumulation and FeP reductive dissolution, which were mainly driven by the coupled factors of anoxic sediment condition, SRBs and FRBs activities. In addition, PO₄³⁻ diffusion from pore water to overlying water in the east of the lake may be prevented for its much higher Fe/P ratio (8.06) and Fe²⁺ concentrations in pore water, which may form a P-adsorbing barrier of iron oxides in the interface between pore water and overlying water. This study enhances the understanding of the vicious P cycle pattern in the aquatic environment driven by algae bloom and decomposition, which should be considered when conducting eutrophication prevention and control measures on lakes.

It is a two-way interaction between algae bloom events and nutrient cycles in aquatic environments. In Meiliang bay of Taihu Lake, phosphorus (P) forms in the water, sediment and pore water, and bacterial community structures in the sediment were investigated in June 2021 (the algae bloom period) and December 2021 (the algae collapse period). The aim of this study is to clarify the periodic variations of P migration and transformation driven by algae bloom and collapse. Results showed that concentrations of total P and total particulate P in the water during the algae bloom period (.13–.25 mg/L) were much higher than those during the algae collapse period (0–.13 mg/L), which was mainly caused by the uptake of phosphate (PO₄^3-) by algae in the surface water. Compared with the algae bloom period, there were higher concentrations of organic P (OP), iron-bound P (FeP) and inorganic P in the sediments during the algae collapse period. The propositions of OP and FeP in total P in the sediments increased from 19% to 17% during the algae bloom period to 27% and 33% during the algae collapse period. These suggest the cumulative trend of OP and FeP in the sediments during the algae collapse period, and FeP might be formed through the processes of OP mineralization and P adsorption by iron oxides/hydroxides in the sediments. Different routes of sediment P regeneration existed over the two periods. During the algae bloom period, the similar vertical variations of labile PO₄^3- and labile Fe²⁺ in the sediments provided in situ, high-resolution evidence for FeP reductive dissolution driven by FRBs activities. During the algae collapse period, OP mineralization driven by organic P-solubilizing bacteria activities and accelerated by the sulfate reduction process was confirmed by the similar vertical variations of labile PO₄^3- and labile S^2- in the sediments. Therefore, treatment approaches and management practices should consider the periodic variations of internal P cycles in aquatic environments during the algae bloom and algae collapse periods to avoid inefficient treatments of lake eutrophication and algae bloom.

Reservoir water level regulation induces intricate processes of phosphorus (P) migration and release within the water-level fluctuation zone (WLFZ). These dynamic interactions pose significant challenges for effective pollution management strategies. This study focused on two typical tributary WLFZs (narrow gorges type and wide river type) in Three Gorges Reservoir (TGR), aiming to quantify P sources and assess their release potentials across two operational periods. Results showed that the deposition of suspended particulate P (SS-P) transported from the upstream was the dominant P source in the two WLFZs during the low water level period (August 2022). During the drainage period, the main P source of the ‘narrow gorges type’ WLFZ was P loss from slope soil above the WLFZ, but the ‘wide river type’ WLFZ had a mix of P sources, including P loss from slope soil, SS-P depositions from the TGR backwater and the tributary upstream. Among the three P sources, the slope soil source exhibited a relatively higher degree of P saturation (DPS) values (0.5–18.8%), indicating a greater potential for P release. Given that P loss from slope soil is the primary P source in the WLFZs during the drainage period, which coincides with the spring plowing season, it is crucial to implement measures to prevent P loss from slope soils to safeguard water quality in the TGR.